Embarrassing question about Hfe ...

[solarplexus]

Ok... I know how Hfe works in a circuit concerning gain and clipping... but what DOES the term HFE mean ?

Tried to find it in GEOFX website and in the search function... no luck.

I am a little embarassed asking this question, even more when I understand how it works, so I should know what it means...

Anyway... thanks for your help!

Matt

[modsquad]

...............Ic...............Ie
Hfe ~= ---------- ~= ---------
...............Ib...............Ib

The formula for hfe refers to the ratio of collector current (Ic) to base current (Ib).

[solarplexus]

thank

but what does Hfe MEANS like... V=voltage, a=ampere... etc... 

[Sir H C]

It is related to a type of two-port network representation of the transistor, the hybrid model.  http://ee.stlcc.info/234/hybrid.htm

[mac]

thank
but what does Hfe MEANS like... V=voltage, a=ampere... etc... 

hFE has no units, since hFE = ic/ib == [ampere/ampere] = [1].

mac

[solarplexus]

ok so from what I can read, Hfe is the forward current gain of the transistor??

[modsquad]

Yes...Its the current gain of the transistor

[solarplexus]

Alright.

Thanks everyone!  I feel better now.

[R.G.]

The term "hfe" comes from one of the early schemes to determine a good way to describe a transistor; this was not at all obvious when they were first made up.

"H" comes from "hybrid parameters", a set of descriptions of what happens to one thing you measure as you vary another one.

"F" means "forward"
"E" means "emitter grounded".

So hfe stands for hybrid forward gain, emitter grounded.

the others are hie (input resistance, emitter grounded), hoe (output resistance, emitter grounded) and ... um... dang, we never used that other one. "Hre"? It escapes me now. I haven't dealt with other than hfe for decades.

[solarplexus]

so let's say in a FF, to cut off RF interference, you add a 10k as the Hie?

[mac]

If you mean the 10k or pot in the Axis Face, I use it to avoid the tone sucking when I use the HB of my guitar, or put a wah or the rangemaster in front of my FF. To stop RF with my Si version I put some pico farads from base to gnd.

Hey, I lke that... hIE

BTW, hRE... ¿hybrid, reverse gain, emiter gnd? ¿ib/ic?   

mac

[solarplexus]

I'm sorry it's not a 10k but a 1k resistor as suggested in the Tantalus project at ROG

http://www.runoffgroove.com/tantalus.html

[brett]

The H indicates that it is a "parameter" or characteristic of the device that we are concerned with.
So we aren't talking about any external components, whether resistors or caps or whatever..
I think Hie will be a few kohms for old germanium devices, and up to a few hundred kohms for high hfe silicon ones. 
You can calculate it (approximately) from the resistance of the base-emitter junction x the current gain through the base.  There's some formula like 25mA/mV (??) to calculate the base-emitter resistance for forward-biased silicon junctions.
hope this helps

[R.G.]

All of the H parameters are actually curves, and properly should be represented as graphs.

hie is close to but not exactly the same as the Shockley resistance (which is the 25mv/Ie that you're talking about) times the current gain. Shockley came up with his resistance based on empirical tests of the first transistors. H parameters came later, I think.

I have a book somewhere in the boxes with that bit of history in it, I think.

[Satchmoedie]

 Hfe is forward gain. We also have Hie (input Z), Hoe, (output Z) and Hre which is  very low and usually assumed to be zero and thus usually ignored and thus usually forgotten. So unless you are doping crystals and have a transistor factory in your basement that Hre is hardly ever taken into account. They do make ultra sensitive bench meters that will give pico ohms and .000001 pico farads readings and are limited to this type of work as their top range is useless for what we do. Hre is the transistor's dependence on the curve of Ie-Ibe on Vce. I saw an LCR for sale with a curve tracer that would read this parameter, but it was not good for much else except mfg of semiconductors. I do not think it ever sold either. So Hie is input impedance and Hoe is output impedance and Hfe is forward gain. It is similar to the mu of a vacuum tube I suppose. Now we all know the hybrid circuit parameters, and what they can be used for. (I GUESS) I have a transistor curve tracer, but it sure as heck won't do Hre.  My Radio Shack multimeter that has a 1KHz oscillator to read capacitance and also does HFE is great. Harbor Freight sells one that does the Hfe, but you must know which pin is which. My Radio Shack one tells me it is NPN, or PNP and what the pin out is. I love it! I got it used at a thrift store for $20. Definitely a great investment. The $8 Harbor Freight one is OK, but you have to know the pin out. 

[PRR]

Late to the party?

> Hfe is forward gain. It is similar to the mu of a vacuum tube I suppose

No.

The "hIE" of a negative-grid vacuum tube at low frequency is "almost infinity". Finite output current with nearly zero input current. Perhaps 1mA out for 0.01uA in, "hFE"= 100,000.

hOE is the "Mu" of a BJT. hOE is typically far over 1,000, far greater than the device's in-circuit voltage gain in typical circuits; therefore it is typically as meaningless as a pentode's Mu (~~500).

------------------------

> Shockley came up with his resistance based on empirical tests of the first transistors

Unlikely. While there was a trend, the early (1947) point-contact transistors were so awful that no firm theory was ever developed. OTOH Shockley was a brilliant thinker, and seems to have perfected the Junction transistor in his mind (Jan 1948) before fabrication was possible (1949-1951).

Shockley's Law "can" be deduced from pure reason and universal constants: drift, diffusion, and thermal recombination, times area (Is). It is analogous to gas law (pressure, volume, temp). He probably knew A.H. Wilson's paper of 1932, which gets into an exponential of 1/kT, and finds a value of 0.3V for cuprous oxide (similar to 0.026V for Silicon).

[DavenPaget]

thank

but what does Hfe MEANS like... V=voltage, a=ampere... etc... 

Simply speaking , current gain , Collector I/ Base I or A
It's usually written as Current Gain in datasheets .
Like 2N5089 has a min. current gain of 400 and max of 1200

[mac]

Shockley voltage is close to:
kT/e = 0.025875V @ 300K
where k is Boltzmann constant, 1.38e-23  J/K [Joule/Kelvin degrees], T the temparature in Kelvin degrees, and e the electron charge 1.6e-19 C.

The term kT, which is always found in Statistical Physics, can be thought as the particle energy in a sea of other particles... rougly speaking.
Think of an ideal gas, since particles do not interact, the total energy is NkT, N: total number of particles.
From this point of view, it looks like the Shockley voltage is the voltage of one electron in a sea of electrons.

Just a thought (I've just open my eyes and I did not wash my face with cold water nor drink coffee yet)

mac